1. Field of the Invention
The present invention relates to apparatus used for the thermal development of photothermographic media. In particular, the present invention relates to a filter for use in such thermal development apparatus.
2. Background of the Invention
Thermographic and photothermographic imaging systems based on the generation of silver images by the thermally induced reduction of silver salts are well known in the art. A silver image is generated by the localized (imagewise distributed) reduction of a silver salt, ordinarily the reduction an organic, low-light sensitivity or light insensitive organic silver salt (usually referred to as a light insensitive silver salt) by a reducing agent for silver ion. In a thermographic system, the differentiation between the image and the background is controlled by imagewise distribution of heat, with the silver image being formed where heat is applied. In a photothermographic system, a light sensitive silver salt (i.e., silver halide) is placed in catalytic proximity to the light insensitive silver salt. When the silver halide is struck by radiation to which it is sensitive or has been spectrally sensitized, metallic silver (unoxidized silver, Ag.degree.) is photolytically formed. The photolytically formed silver acts as a catalyst for the further reduction of silver salt, including the light insensitive silver salt in catalytic proximity to the silver halide. Upon heating of the radiation exposed photothermographic element, the light insensitive silver salt in catalytic proximity to silver halide having developable silver specks thereon are more rapidly reduced by reducing agent which is present around the silver materials. This causes the silver image to be primarily formed where the photothermographic element was irradiated.
The most common type of photothermographic element which is commercially available comprises a silver halide as the light sensitive silver salt (either as in situ formed silver halide or preformed silver halide), a silver salt of an organic acid (usually a salt of a long chain fatty acid (e.g., having carbon lengths of 14 to 30 carbon atoms, such as behenic acid) as the light insensitive silver salt, a photographic silver halide developer or other weak reducing agent as the reducing agent for silver ion, and a binder to hold the active ingredients together in one or two layers (e.g., U.S. Pat. No. 3,457,075).
Development usually occurs by placing the exposed photothermographic element in contact with a heated surface (e.g., a heated roller or platen) or in an inert heated fluid bath. The heated rollers used in the past have generally been fairly open to the environment which has enabled any innocuous materials generated or evaporated by the heating step to harmlessly escape to the atmosphere. Newer types of imaging systems sometimes desire more closed work areas or completely closed systems which do not have ready venting to the atmosphere. It would be a severe limitation on thermal developing units for use with photothermographic elements, if they were to be part of a more closed system, to require a dedicated venting or exhaust system for evaporated materials.
Commercial models of thermal processors for photothermographic elements, such as the 3M Model 259B Continuous Thermal Processor have contained some filtering means on the equipment. In that particular processor, the filtering means is separated from the actual thermal development area of the processor as shown in the Illustrated Parts Manual for that processor. This filter acts to capture airborne condensate formed from material evaporated from the thermally developed media.
It has been found by the inventors that thermal development of photothermographic elements in a closed imaging unit allows for certain harmless materials evaporated during the thermal development step to deposit on the interior of the unit. This condensation of materials (e.g., such as the free fatty acid generated upon reduction of the silver salt and then evaporated during development) can adversely affect many aspects of the imaging process. The condensation may clog vents and cause the developer unit to overheat. The condensate may deposit on the heating element and cause localized insulation of the heated surface in a random fashion, producing image variations across the imaged element. Deposits on the pressure rollers can also lend to image variation from differential heating or can cause marking (pressure marking or transfer deposition) on the film. Electronic components can fail due to corrosion when exposed to released vapors. The condensate may deposit on or be transferred to imaging media or on seams of the unit and cause an unsightly appearance or leave greasy materials on the hands of anyone using the unit. It was necessary to find a means of removing the evaporated materials from the vent stream without the need of a dedicated vent (e.g., a vent that accesses the exterior of a room or building or a special ducted vent stream within a building).